The present disclosure relates to a color splitting/combining prism that is provided in a projector. More particularly, the present disclosure relates to a color splitting/combining prism suitable for a projector that employs a laser light source as an illumination light source, and relates also to an optical system and a projector that employ such a color splitting/combining prism.
There are conventionally known projectors that employ image display elements such as liquid crystal display panels and DMDs (digital micro-mirror devices) as devices for projecting digital information such as video stored in recording devices and documents and images stored in personal computers. A projector comprises an illumination light source that radiates illumination light, an illumination optical system that directs the illumination light from the illumination light source to an image display element, and a projection optical system that directs the projection light from the image display element to a projection lens.
The illumination light emitted from the illumination light source is directed via the illumination optical system to be split apart into light of three colors of red, blue, and green, and is then shone on image display elements corresponding to those colors respectively. The illumination light shone on the image display elements emerges from them as projection light, which is then combined back together, and is then directed through the projection optical system to the projection lens to be projected onto a screen.
To split apart and combine together illumination light, a color splitting/combining prism is used. Thus, a color splitting/combining prism is an optical component that is used in both the optical system that directs illumination light and the optical system that directs the projection light, and is composed of a plurality of prisms each in the shape of a triangular prism. For the purpose of splitting apart and combining together light, dichroic films are formed on predetermined faces. Via those dichroic films, light of predetermined wavelengths is reflected and light of other wavelengths is transmitted, and thereby the illumination light is split apart and combined together.
As illumination light sources, high-intensity xenon lamps and high-pressure mercury lamps are conventionally used. Today, laser light sources are in practical use that employ LEDs (light-emitting diodes) and semiconductor lasers as these have come to offer improved emission efficiency and increased emission intensity. In particular, after the advent of blue laser light sources, there have been developed illumination light sources that employ laser light sources of three primary colors, namely a blue laser light source combined with a green and a red laser light source.
In a projector that employs a DMD, OFF-light (unnecessary light) is produced as a result of, at pixels at which a particular color is not to be projected, illumination light being reflected in the direction opposite to that of the projection optical path. The OFF-light is supposed to be expelled from the front face of a color splitting/combining prism to outside the projection optical path. However, the characteristics at the angle at which the OFF-light is incident on a dichroic film differ from those for the projection light (ON-light) due to angle dependence of the dichroic film; thus, the OFF-light may be directed in an unintended direction, such as upward, instead of to the front face of the color splitting/combining prism.
Thus, with illumination that employs a laser light source that produces extremely intense light compared with ordinary lamp light, the OFF-light may be discharged in an unintended direction, raising the temperature of components nearby and becoming stray light which is directed to the projection lens to cause ghosts and low contrast.
There has already been proposed, in Japanese Patent Application Publication No. 2000-258725, a projection optical system in which, of a dichroic surface on a color splitting/combining prism, a region struck by ON-light is provided with a dichroic film having characteristics optimized for rays having an incidence angle similar to that of ON-light and a region struck by OFF-light is provided with a dichroic film having characteristics optimized for rays having an incidence angle similar to that of OFF-light.
A common dichroic film has angle dependence; by providing, instead, a dichroic film that exhibits predetermined characteristics at a particular incidence angle, it is possible to reflect light in a predetermined wavelength band satisfactorily and transmit other light satisfactorily.
In the optical system disclosed in Japanese Patent Application Publication No. 2000-258725, in both of the regions of the dichroic surface that are struck by ON-light and OFF-light respectively, the characteristics with respect to light corresponding to the respective incidence angles are adjusted. Thus, when ON-light and OFF-light are incident at predetermined angles respectively, desired characteristics are exhibited so that satisfactory reflectances and transmittance can be obtained. However, it is not easy to divide into a plurality of regions and form a plurality of different dichroic films, and doing so is disadvantages also in terms of production cost. In addition, if given light is incident at a position away from a predetermined region, desired characteristics may not be obtained.
Thus, in a color splitting/combining prism, a dichroic surface is required not only to achieve satisfactory wavelength separation with respect to projection light as a principal light beam but also to achieve satisfactory wavelength separation even when struck by OFF-light with a larger incidence angle than projection light. In other words, a dichroic surface is required to achieve wavelength separation for all of a plurality of light beams that each have a different incidence angle and a predetermined spread angle.
In particular, in a case where high-output laser light is used, if OFF-light, which is incident at a still larger angle, becomes stray light, not only does light leaking in the direction of projection light degrade imaging performance, but also light reflected elsewhere than toward a prism optical surface may be absorbed by adhesive and the like, producing heat and hence unnecessary damage.
The present disclosure is directed to a projector provided with an illumination light source that radiates laser light and a color splitting/combining prism, and a color splitting/combining prism, and an optical system and a projector that employ it, that can reduce unnecessary reflected light, prevent damage to optical components and adhesive, suppress image degradation due to stray light, and enhance light use efficiency.